Interfacially bonded structures or laminations fabricated from two or more oriented organic polymer films, sheets or strips in recent years have found widespread use in a variety of applications by virtue of the high level of mechanical properties that can be imparted to these components by orientation. Further exploitation of increasingly high levels of properties, however, has been hampered to some extent by certain limitations imposed by production methods and certain inherent characteristics of the materials themselves.
In this regard, biaxially oriented films of a good balance of properties can be produced, but in an effort to make biaxially oriented films of very high tensile strengths by sequential drawing, the high strength attained in the first direction draw is diminished by the second direction draw. Uniaxially oriented films have a very high level of tensile strength in the direction of orientation, but strength in the orthogonal direction is very low. Such films (especially those which are heat set) can fibrillate so easily that utility, even in applications primarily dependent upon the tensile strength in one direction, is impaired.
Cross-lapped structures of uniaxially oriented films, with the direction of orientation of adjacent layers at an angle to each other to utilize the very high tensile strengths of this type film have met with only limited success. This limitation is primarily caused by the inability to make fully satisfactory interlaminar bonds, the poor bonding resulting in delamination failures of the laminated structure.
Highly oriented films of condensation polymers such as polyethylene terephthalate (PET), are very poorly receptive to adhesives; only a very few complex costly adhesives can be used, and only after pretreatment of the film to render it more adherable. Adhesives commonly used, furthermore, have very low strength in bulk and in thin layer in comparison to the oriented polymer. Adhesively bonded structures, accordingly, cannot make optimum use of the potential of cross-lapped structures of uniaxially oriented films.
Likewise, fusion bonding of cross-lapped structures of uniaxially oriented film is less than satisfactory in capitalizing on the potential of such structures. In thermoplastic films of major commercial importance, such as representative condensation polymers PET, poly(butylene terephthalate) (PBT), the aliphatic polyamides (e.g., nylon 6 and nylon 66) and polyolefin addition polymers, fusion destroys orientation. In the art, U.S. Pat. No. 4,384,016 discloses fusion bonding of polymers which form liquid crystals in the melt. These are described as wholly aromatic condensation polymers. Any orientation imparted to these polymers, usually formed in melt drawing, is not only retained upon melting, but survives upon returning to the solid state. Polymers to which the present invention is directed are understood to not retain their oriented structure upon melting, but at best form collapsed coils. Accordingly, for the polymers covered by the present invention, fusion bonding is generally unsatisfactory.